This invention relates generally to space vehicles, and, more particularly, to spacecraft for carrying a payload, such as a satellite or space probe, from a first planetary orbit to a second operational orbit in which the payload will be used.
Since the inception of space flight, spacecraft such as communication satellites have been launched by means of expendable, rocket-powered vehicles, which were initially adaptations of vehicles used to carry ballistic missiles. These expendable rocket vehicles have imposed severe weight and size limitations on satellites and similar spacecraft. The same limitations will no longer apply, however, when a launch vehicle known as the space transportation system, or space shuttle, becomes operational. The space shuttle is a reusable vehicle about the size of a small jet airliner. It will be launched in a vertical orientation, like a rocket, using auxilliary rocket engines strapped to its fuselage, and will carry one or more satellites or other payloads in a payload bay up to a circular parking orbit at an altitude of approximately 160 nautical miles (300 km). For the return flight, the shuttle will first be slowed down by its own rocket engines, will re-enter the earth's atmosphere, and will land as a glider on a relatively long runway.
Although the space shuttle removes long-standing constrains on the weight and size of satellites, it also imposes new constraints, and poses new problems with respect to transporting a satellite or other spacecraft from the shuttle orbit to its operational orbit. The payload bay of the space shuttle is a generally cylindrical space approximately 60 feet (18.3 meters) long and 15 feet (4.6 meters) in diameter, and the cargo carrying capacity is approximately 65,000 pounds (29,480 kg). Typically, more than one user will share the shuttle on a single flight, and the cost of the mission to each user is determined from the greater of the weight ratio and the length ratio utilized by the user.
For example, if a user's payload, including an upper-stage propulsion system or transfer vehicle, occupied 15 feet (4.6 meters) of the length of the payload bay, and weighed 15,000 pounds (6,804 kg), the length ratio would be 15 divided by 60, or 25%, and the weight ratio would be 15,000 divided by 65,000, or approximately 23%. Accordingly, the user cost would be determined by the length ratio of 25%.
Clearly, the use of this cost formula encourages spacecraft designers to make the length and weight ratios approximately equal. For most satellite designs, the end result is that the satellite and accompanying propulsion system must usually be designed to be as short as possible to make best use of the payload bay.
An even more important consideration is that the space shuttle will provide transportation to only a relatively low altitude of approximately 160 nautical miles (300 km). Most satellite missions require payloads to be transported to much higher orbits than this. Communication satellites, for example, typically operate in a geosynchronous orbit, having a twenty-four-hour period of rotation, at approximately 19,000 nautical miles (35,000 km) altitude. Accordingly, a propulsion system must be provided to transport a payload, such as a satellite, from the parking orbit of the space shuttle to a higher operational orbit in which the satellite will operate.
Various configurations have been suggested for propulsion systems to transport such a payload to its operational orbit from the space shuttle. Prior to the present invention, however, all of the proposed propulsion systems have included a number of features that are disadvantages in the context of space shuttle operations. Basically, the designs suggested to date for transporting a satellite to its operational orbit all utilize essentially the same principles as an upper stage launch vehicle of the type widely used prior to the space shuttle. These upper stages, or payload assist modules as they are sometimes called, typically have a relatively large solid-propellant rocket engine, which is initially burned at perigee, the lowest altitude point in an elliptical transfer orbit, transferring the payload to an orbit having an apogee at the desired operational orbit altitude. Either the same engine, or a separate one on the satellite itself, is then burned at apogee to circularize the orbit at the desired altitude.
The principal disadvantage of this technique is that there is considerable unnecessary avionic equipment redundancy with respect to the upper-stage or payload assist module and the payload or satellite module itself. Each module is usually designed to be self-sufficient in terms of "housekeeping" functions, such as power supply, attitude and course control, telemetry and communication. The upper-stage or payload assist module functions as a self-sufficient spacecraft while in transition between the space shuttle orbit and the operational orbit, after which the satellite or payload module then also functions as a self-sufficient spacecraft, and may in fact be separated from the upper-stage module on arrival in the operational orbit. It will be apparent that this duplication of subsystems is extremely costly. It has apparently been thought to be necessary, however, to accommodate a wide range of missions for which the space shuttle will be utilized.
Another feature that all of the upper-stage module designs so far suggested have in common is that the ascent from the shuttle parking orbit to the satellite operational orbit is made with basically only two rocket engine burns. While this technique is known to be the most efficient from a fuel consumption standpoint, it imposes severe design constraints on the satellite, since relatively fragile structural components, such as communication antennas or solar cell arrays, must then be able to withstand substantial acceleration stresses if these components are to be deployed before departure from the vicinity of the space shuttle.
It will be appreciated from the foregoing that there is presently a clear need for a transfer vehicle suitable for transporting payloads, such as satellites, from a low parking orbit to a higher operational orbit without any of the aforementioned disadvantages. Ideally, such a transfer vehicle should also be easily adaptable to a variety of mission-specific requirements, and should provide a "soft ride" utilizing relatively low accelerations, so that satellite components can be safely deployed before departure from the vicinity of the space shuttle. The present invention is directed to these ends.